As a thermosetting coating composition, resin compositions utilizing an aminoformaldehyde resin as a hardener (curing agent) are hitherto commonly known. In recent years, the research for changing to a paint or coating material dilutable with water for the purpose of reduction of an organic solvent emitted at the time of coating film drying from the viewpoint of air pollution and environmental protection is becoming active. Simultaneously, an aqueous thermosetting composition that is soluble in water and exhibits high performance is increasingly demanded.
Urethane resins due to the properties thereof of being excellent in mechanical properties, abrasion resistance to base material, flexibility, solvent resistance, etc. find wide applications in paints, inks, adhesives, coating agents, fiber treating agents, etc. On the other hand, amino resins due to the properties thereof of being excellent in appearance characteristic, antiweatherability, mechanical properties, etc. find applications in paints, inks, adhesives, coating agents, fiber treating agents, etc. Paints or coating materials taking advantage of these excellent properties of these amino resin and urethane resin realize surprisingly high performance. The amino resin and urethane resin have been applied through combining of the resins as separate two-pack hardeners.
In the event of mixing of the two-pack liquids into one-pack liquid, it is difficult to attain homogeneous stable storage thereof because of a difference in compatibilities between the amino resin and urethane resin. Accordingly, in conformity with individual usages, it has been necessary to add a third component, such as a stabilizer or a compatibilizer, to such one-pack liquid. However, as the third component tends to adversely affect the performance in each usage, it has been difficult to achieve the exertion of the respective inherent performances of the urethane resin and amino resin.
In efforts for solving this problem, there can be mentioned patent references (1) and (2) proposing the technology of an amino resin excelling in not only the hardenability but also the water resistance, alkali resistance, processability, hardness, etc. According to the description of these references, in the process of amino resin production, a urethane resin is used as a raw material of amino resin so that the urethane resin is incorporated in the structure as a part of the amino resin skeleton to thereby provide a hardener capable of water solubilization. However, in the examples of these references, as a hydrophilic group such as carboxylic acid or an amino group is introduced as an indispensable structure in the ends of the urethane resin skeleton, not only is the water resistance deteriorated but also the multinuclear structure being a characteristic of the amino resin is scattered by urethane structure segments, thereby causing the exertion of appearance feature, high hardness, etc. being the inherent characteristics of the amino resin to be difficult. Moreover, the use of the urethane resin is exclusively for the purpose of water solubilization, and the exertion of toughness, etc. of urethane resin having been regarded as being inherently superior has been unsatisfactory.
Patent reference (1): JP 6-329876 A
Patent reference (2): JP 6-239949 A.